1. Field of the invention
The present invention refers to extrusion presses, particularly but not exclusively presses for the extrusion of metals and more especially for the extrusion of aluminium.
2. Description of the Related Art
These presses traditionally comprise at least an extrusion stem, also known as a pusher or plunger, which is actuated by the cylinder of the press, a container having an inner axial chamber or bore for receiving billets, and an extrusion die downstream of the container. A billet of material to be extruded, carried onto the longitudinal axis of the press by any loading means whatsoever, is loaded into the chamber of the container, which is positioned against the die plate, and then the plunger enters the chamber of the container, applies pressure to the billet and extrudes it through an opening or openings of the die.
The process which has been briefly described above has the drawback that air pockets or blisters often remain trapped in the finished product, with the result that some lengths of a produced section bar may have to be scrapped. These air blisters are due to various causes. In the first place, the inner chamber of the container has a rather wide clearance with respect to the transverse dimension of the billet. In addition, part of the air remains trapped between the irregularities on the surface of the billet. Moreover, air may be trapped between the head of the billet and the die plate if the surfaces in contact are not perfectly smooth and coplanar.
When pressure is applied to the billet by means of the plunger, the billet usually buckles first at the back part (that is the end towards the plunger) until it seals against the walls of the chamber of the container, and therefore it tends to enclose the air inside the chamber, preventing it from escaping to the outside. This entrapment has always been a problem with these types of presses.
Attempts have been made so far to remedy this problem in various ways.
According to one traditional method, the billet is slowly upset until it widens out against the inside surface of the cavity of the container, enclosing a part of the air which is compressed (in practice, pressure is applied to the billet until a preset pressure value is reached); then the pusher and the container make a small move backwards, in such a way that the compressed air flows out towards the atmosphere through the airspace formed between the container and the die; the container is once more brought against the die and the plunger is moved forward until the extrusion of the billet is completed.
This procedure has the advantage of eliminating a great part of the air trapped between the billet and the container; yet it does not succeed in eliminating it completely. In addition, the time required is three or four seconds, which is excessively disadvantageous when it is desired to optimize the whole working cycle of the press; finally, the fact of having to make the machine work in a discontinuous manner subjects the various machine members to additional stress and, in fact, means that the machine members have to be oversized in consideration of this great stress.
Another known system which attempts to avoid the formation of blisters in the finished product consists in pre-heating the billet in a differentiated manner along its length, that is in such a way that the head or front part of the billet, i.e. the end part towards the die, is at a higher temperature than the tail or back end part of the billet, that is the part towards the plunger ("conical heating"). This "conical heating" allows the billet in the container, under the pressure created by the plunger, to widen out until it abuts the wall of the container, first at the front part and then gradually along its body as far as the back part. This system allows the air which is between the sides of the billet and the container wall to be expelled; however, it does not expel any air which may be enclosed between the head of the billet and the die. In addition, the installation costs for this process are high.
The use is also known of a combination of the two systems described above.
In a recently published European patent application Ser. No. 397,473, a container chamber is depressurized before beginning extrusion, through a suction opening in the plunger, said opening being connected, by means of a channel extending axially in the plunger, to a vacuum chamber or apparatus for creating a vacuum. In particular, the publication describes a plunger provided with a pressure pad on the plunger foot, this pad being carried in a mobile way on the end of the foot, in such a way that it leaves between the pad, when it is in the extended position, and the foot, a passage which communicates with an axial channel, said passage closing when the pad is pressed against the foot of the plunger during the thrust on the billet.
One drawback with this system lies in the fact that the plunger with a pad is expensive to produce; in addition, it is well known that the entire end part of the plunger, called the foot, is subject to wear and has to be replaced relatively often; the frequent replacement of the foot with a pad therefore makes this equipment expensive to run.
Yet again, since the suction is carried out without there being any effective seal between the plunger and the inside wall of the container, it is easy for the outside air to seep in around the foot of the plunger, making the vacuum in the container less efficient. Finally, suction ceases the moment when the force exercised between the plunger and the billet becomes greater than the force of the spring which moves the pad away from the foot of the plunger, and therefore in these conditions the pad returns to its housing on the plunger, closing the annular suction opening.